Additional Proxy Soliciting Materials (Form DEFA14A)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrant ☒ Filed by a party other than the Registrant ☐

Check the appropriate box:
☐ Preliminary Proxy Statement
☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))
☐ Definitive Proxy Statement
☒ Definitive Additional Materials
☐ Soliciting Material Pursuant to §240.14a-12

REGENXBIO Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

☒ No fee required

☐ Fee paid previously with preliminary materials

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rule 14a-6(i)(1) and 0-11

Your Vote Counts! REGENXBIO INC. 2026 Annual Meeting Vote by May 28, 2026 11:59 PM Eastern Time REGENXBIO® REGENXBIO INC 9804 MEDICAL CENTER DRIVE ROCKVILLE, MD 20850 V92514-P51733 You invested in REGENXBIO INC. and it's time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on May 29, 2026. Get informed before you vote View the Notice and Proxy Statement and Annual Report online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 15, 2026. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to [email protected]. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. P For complete information and to vote, visit www.ProxyVote.com Control # Smartphone users Vote in Person at the Meeting* Point your camera here and May 29, 2026 vote without entering a 9:00 a.m. Eastern Time control number 9804 Medical Center Drive Rockville, Maryland 20850 *Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares.

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is only an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Board Voting Items Recommends 1. Election of three Class Il Directors Nominees: 01) Jean Bennett, M.D., Ph.D For All 02) A.N. "Jerry" Karabelas, Ph.D. 03) Daniel Tassé 2. To ratify the appointment of PricewaterhouseCoopers LLP by the Audit Committee of the Board of Directors as the Company's independent registered public accounting firm for the year ending December 31, 2026. v For 3. Advisory vote on the compensation paid to the Company's executive employees. ~ For 4. To approve a stock option exchange for our non-executive employees. For 5. To approve a stock option exchange for our executive employees. For NOTE: Other business may be considered as may properly come before the meeting and any adjournment or postponement thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click "Delivery Settings". V92515-P51733